Present embodiments relate generally to gas turbine engines. More particularly, but not by way of limitation, present embodiments relate to resistive temperature detectors for exhaust gas temperature measurement in a gas turbine engine.
In turbine engines, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gas which flow downstream through turbine stages. These turbine stages extract energy from the combustion gas. A high pressure turbine includes a first stage nozzle and a rotor assembly including a disk and a plurality of turbine blades. The turbine engine may include a number of stages of static airfoils, commonly referred to as vanes, interspaced in the engine axial direction between rotating airfoils commonly referred to as blades. The high pressure turbine first receives the hot combustion gas from the combustor and includes a first stage stator nozzle that directs the combustion gas downstream through a row of high pressure turbine rotor blades extending radially outwardly from a first rotor disk. In a two stage turbine for example, a second stage stator nozzle is positioned downstream of the first stage blades followed in turn by a row of second stage turbine blades extending radially outwardly from a second rotor disk. The stator nozzles direct the hot combustion gas in a manner to maximize extraction at the adjacent downstream turbine blades.
The first and second rotor disks are joined to the high pressure compressor by a corresponding rotor shaft for powering the compressor during operation. A multi-stage low pressure turbine follows the two stage high pressure turbine and is typically joined by a second shaft to a fan or low pressure compressor disposed upstream from the high pressure compressor in a typical turbofan aircraft engine configuration for powering an aircraft in flight.
As the combustion gas flows downstream through the turbine stages, energy is extracted therefrom and the pressure of the combustion gas is reduced. The combustion gas is used to power the compressors as well as a turbine output shaft for power and marine use or provide thrust in aviation usage. In this way, fuel energy is converted to mechanical energy of the rotating shaft to power the compressor and supply compressed air needed to continue the process.
During the operation of the gas turbine engine, it is necessary to obtain temperature readings at different locations in the engine. This data is utilized by the engine control logic to properly operate the engine and provide maximum performance at the highest efficiency. One such temperature probe which is utilized at the exhaust area of the combustor, it is known as an Exhaust Gas Temperature probe or EGT probe or sensor. These probes utilize type-K thermocouples typically having dissimilar metals to create a differential which may be then input to the engine control logic to optimize performance.
Resistance temperature detectors (RTD) are also utilized in probe assemblies to measure operating temperatures. RTDs utilize variable resistant material at a position where a temperature is to be measured with leads connected to an instrument which measures an amount of varying voltage when power is supplied to a sensor. Since resistance changes with temperature, the temperature may be determined by applying a constant current to the resistor and measuring the voltage drop to determine the resistance and resultant temperature.
Various metals may be used which provide differing resistances upon exposure to heat. One problem with use at high operating temperatures is that materials suffer from oxidation, ionic migration wire alpha shifts and weakened strength. This can result in decreased life as well as temperature measurement drift. For operation in high temperature environments, typically at or above 1832 degrees Fahrenheit, operating conditions are limited to specialized RTD constructions using platinum, wire wound or thin film constructions. However, even these constructions are still limited generally for short durations of temperature exposure above 1832 degrees Fahrenheit which severely limits applicability in various industries.
Current designs cannot withstand temperatures at the combustor exit or high pressure turbine entrance for extended periods of time. Accordingly, temperatures must be taken near the low pressure turbine and extrapolated to a position at the combustor exit. This however can lead to error. Additional problems occur such as decreases in engine management efficiency due to the extrapolation, as opposed to obtaining an actual reading. Moreover, time delays in optimization of engine conditions may further result in less than maximum engine performance when engine operating conditions change, for example ambient air temperature changes. For these reasons, a compromise is struck between positioning too close to the combustor, which may result in early failure of the detector, and placing further from the combustor which may result in less accurate temperature readings at the combustor exit.
As may be seen from the foregoing, there is a need to optimize the engine management by providing a temperature reading closer to the combustor. There is a further need to optimize temperature detectors so that the temperature detector can withstand temperatures typically occurring at the combustor and high pressure turbine and inhibiting degradation, drift and failure at high temperatures for extended periods of time.